The present disclosure generally relates to subterranean formation operations, and, more specifically, selective deactivation and activation of permeability modifiers for use in subterranean formation operations.
The production of water with hydrocarbons (e.g., oil) from subterranean wells constitutes a major problem and expense in the production of the hydrocarbons. While hydrocarbon-producing wells are usually completed in hydrocarbon-bearing formations, such formations may contain, or may be adjacent to, water-bearing sections. Generally, the term “water-bearing section,” and grammatical variants thereof, refers to any portion of a subterranean formation that may produce water, including a hydrocarbon-bearing section that has sufficiently high water saturation such that water may be produced along with hydrocarbons. The high mobility of water in a subterranean formation may allow it to flow into a wellbore therein by way of natural fractures and/or high permeability streaks present in the formation. Over the life of such wellbores, the ratio of water to hydrocarbons recovered may be undesirable in view of the cost of producing the water, separating it from the hydrocarbons, and disposing of it, which can represent a significant economic loss.
Subterranean stimulation treatments may be employed to increase the flow of hydrocarbons into a wellbore. One such stimulation treatment is hydraulic fracturing, where treatment fluids are pumped into the subterranean formation at a rate and pressure sufficient to create or enhance at least one fracture therein, thereby increasing fluid flow through the formation into a wellbore therein. Proppant packs and/or gravel packs may be formed within the formation to further facilitate fluid flow into the wellbore. When a formation contains water-bearing sections, however, stimulation may lead to the undesired, increased production of water with the hydrocarbons.
Another subterranean stimulation treatment is acid stimulation (e.g., “acidizing”), in which an aqueous treatment fluid comprising an acid is introduced into the formation to dissolve acid-soluble materials that may clog or constrict formation channels, thereby potentially widening the pathways through which hydrocarbons may flow from the formation into a wellbore. Acid stimulation treatments also may facilitate the flow of injected treatment fluids from the well bore into the formation. In certain circumstances, however, the acidizing treatment fluids may undesirably enter the water-bearing sections instead of the hydrocarbon-bearing sections in the formation because the water-bearing sections may be more permeable to the aqueous acidizing treatment fluid than the hydrocarbon-bearing sections. Thus, acid stimulation treatments may result in an undesirable increase in the production of water.
During drilling and/or stimulation of a subterranean formation, water may undesirably enter into a wellbore formed or being formed therein, as described. However, the treatment fluids used in such subterranean formation operations, and others, may also become lost in the subterranean formation in high permeable zones, for example. This “fluid loss” may result in a buildup of solid materials (e.g., proppant, weighting agents, and the like), or a “filter cake,” against a face of the wellbore, which may result in ineffective treatment fluids, a decline in produced fluid flow, formation damage, and the like. Accordingly, the flow of water or other non-produced fluids into or out of the formation may result in significant remedial measures, reduced production, and the like.
Permeability modifiers, also termed “relative permeability modifiers” have been effective at decreasing the production of water with hydrocarbons, and decreasing the fluid loss into a formation, and are used in a variety of subterranean formation operations including, but not limited to, drilling, stimulation, water control, acid diversion, injection fluid profile modification, fluid loss control, gravel packing operations, and the like. The permeability modifiers are capable of altering the relative permeability of a portion of a wellbore that they come into contact with, resulting in blockage of water production and/or diversion of aqueous fluids away from that portion of the wellbore.
However, pressure increases are often encountered during introduction of these permeability modifiers in a treatment fluid through pumping equipment, which limits the amount of such fluid that may be introduced and the depth of penetration. Such a reduced amount of fluid may result in less than desired formation coverage. Because of the pressure increases, injection rates must be reduced to allow placement of sufficient volumes of the fluid comprising the permeability modifier into a formation without breaching the fracture gradient. As used herein, the term “fracture gradient” refers to the pressure necessary to create or enhance at least one fracture in a subterranean formation. Such injection pressure limitations are particularly heightened in subterranean formations having low permeability zones (e.g., less than about 100 millidarcy (mD)).
Additionally, although the permeability modifiers may decrease or prevent water production and fluid loss, during production of the subterranean formation, these permeability modifiers may also impede flow of produced fluids. As such, the permeability modifiers are particularly useful in wellbores where there is no concern that the effects of the permeability modifier (e.g., reduction in water permeability) may remain in effect for a period longer than desired or permanently, but may require removal or otherwise deactivation in hydrocarbon producing wells.